<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>

<body>
    <ul id="box"><li>6</li><li>6</li><li>6</li><li>6</li><li>6</li><li>6</li>
    </ul>
</body>
<script>
        var list = document.getElementsByTagName("li");
        var parents = document.getElementById("box");
        var res = parents.children;
        
        console.log(res);
        for (let i = 0; i < list.length; i++) {
            list[i].innerHTML=parseInt(list[i].innerHTML)*2;
            
        }
    </script>
</html>